The present invention relates to currency verifiers for use in currency changers, vending machines and the like, and particularly to currency verifiers capable of checking color.
Color checking to detect the presence of appropriately colored ink on U.S. or other types of currency has proven to be a useful aid in automated currency verification systems. Most techniques to date which have utilized color checking have depended on arrangements of photodetectors using filters, with such photodetectors arranged in a bridge circuit to attempt to detect color. Such arrangements, however, only achieve a limited degree of sensitivity and can usually be defeated by some shade of gray or colorless marking on the paper at the spot being observed. This is at least partially caused by the fact that in an engraved area of a U.S. bill the green ink lines typically cover only 30 percent or so of the surface, and thus the effect of the ink color on the nature of the reflected light is substantially reduced.
The general condition of the currency or specimen bill being examined is another factor which can affect the results of a color check. If the bill is soiled, the reflection of light from the surface of the bill is reduced. The properties of the reflected light are dependent upon a large number of factors relating to the paper, including its texture and translucence, degree of soiling, and amount of color pigment. The large number of factors affecting the magnitude of the reflected rays tends to mask the effect of the different ink colors and therefore make the detection of any particular color extremely difficult.
One attempt to reduce susceptibility to extraneous factors involved the measurement of light reflected from one point on a bank note with two photocells, one covered with a green filter and the other covered with a red filter. The two photocells were included in a circuit which produced the difference between the two measurements. One such circuit is shown in U.S. Pat. No. 3,496,370 to Haville et al. Because the measured difference values for genuine bills vary widely due to soiling, however, broad tolerance limits are required with this approach.
Recognizing this problem, Mustert, in U.S. Pat. No. 3,679,314, discloses an alternative system which determines the ratio of two readings from a single test point rather than determining the difference value. Mustert found that, since soiling of a bill has substantially nonselective absorption properties, the influence of soiling can be eliminated by taking the ratio of the two measurements. The Mustert apparatus uses rotating mechanical parts to provide the two different colors of light, with two color filters being mounted on a rotating disc in the path of a single light source, or alternatively by having a light beam alternately directed through two stationary filters by a rotating mirror.
Another apparatus, described in U.S. Pat. No. 4,204,765 to Iannadrea et al., tests colored securities with sequentially operated LEDs of various colors directed toward a particular point on the surface of a bill. A single photodetector senses the reflected light of each wavelength. This apparatus does not need external color filters. However, the output signals associated with the different LEDs are supplied to comparator circuitry to determine their relative values, and so wide tolerances are still necessary because of the wide variations in signals from genuine bills.
Phares, in U.S. Pat. No. 3,360,653, compensates for the condition of a test bill by adjusting the voltage level of each test photocell according to the light received by a reference photocell positioned adjacent a clear portion of the bill. The test photocells, which are each associated with a different test area, receive light from a single light source and thus generate one output signal each. Each test photocell is coupled to a window detector which provides an acceptance signal for an output signal within its preset voltage range. A bill is determined to be valid if all window detectors produce acceptance signals, without regard to relative values of different color signals from a single test area or of signals from different areas.
Haville et al., mentioned above, includes a light control circuit which compensates for the condition of the bill by adjusting the intensity of the light source in a pattern-evaluating circuit based on the light received from a dedicated reference photocell. This technique is not applicable without substantial modification to a color detection circuit with two light sources of different colors because of imbalances in intensity which would result from slight differences in the light source characteristics.
Aging and environmental conditions can also adversely affect currency verifier operation. The spectral distribution of the output of a narrowband light source, such as a narrowband LED, often changes significantly over the life of the light source. It has been learned that, in currency verifiers detecting color differences with a pair of light sources, these changes often produce significantly different effects on the two light sources, contributing to errors in bill verification through circuit imbalance. Environmental factors have also been found to cause circuit imbalance. In many areas of the country vending machines and currency changers frequently experience changes in ambient temperature of 30 degrees fahrenheit or more in the course of a day. Such temperature changes can cause a shift in the peak of the spectral distribution or affect the amplitude characteristic of a light source. Output amplitude can also change with dirt or dust on the lens of a light source. These conditions produce an overall reduction in accuracy for existing currency verifiers of this type.